Generally speaking, titanium alloys are higher in strength than pure titanium, and are high in specific strength, fracture toughness, heat resistance and corrosion resistance, and have other excellent properties. Thus titanium alloys have high practical values as materials for aircraft and materials used e.g. in the marine, electric power application and automotive fields.
With an increasing demand for faster and larger aircraft, demand for titanium allows are increasing as materials for primary structural members of aircraft such as outer plating, frames and joint fittings and fastening members (fasteners).
Fasteners such as bolts and nuts are used under harsh conditions where they are repeatedly subjected to various stresses including thermal stress. Because the contact potential difference between titanium alloys and carbon fiber reinforced plastics is small, titanium alloys are less likely to corrode when brought into contact with such plastics. Thus titanium alloys are used as materials of fastening parts (fasteners) such as bolts and nuts for fixing together laminates made of carbon fiber reinforced plastics and forming center wings, main wings and tails of aircraft.
Properties required for such fasteners include predetermined wear resistance as threaded members, and sliding properties in order to ensure necessary tightening torque from the design viewpoint. In order to fulfill these requirements, fastening parts made of titanium alloys have their surfaces subjected to plasma carburizing treatment.
When titanium metal is subjected to plasma carburizing treatment, accelerated activated carbon ions collide against and stick to the surface of the titanium metal, and then diffuse into the titanium metal thereafter. Otherwise, the moment the carbon ions collide against the surface of the titanium metal, they are driven into the titanium metal. In either case, a hard layer of a metal carbide such TiC is formed on the surface of the titanium metal.
Carburizing treatment is performed e.g. by cleaning the surface of titanium metal, and subjecting the titanium metal to plasma carburizing treatment in an atmosphere containing hydrocarbon gas and kept at 0.5 to 15 torr and 700 to 1100° C. so that the titanium metal maintains its inherent strength (see the below-identified Patent document 1).
It is also known, in order to improve the corrosion resistance and the wear resistance, to form a dense film of a titanium oxide on the surface of titanium metal and then subject the surface of the titanium oxide to plasma heat treatment in an atmosphere containing a hydrocarbon gas and kept at 0.1 to 30 torr and 400 to 1100° C., thereby efficiently forming a high-quality glass-like carbon film (see the below-identified Patent document 2).
Glass-like carbon is a non-directional aggregation of hexagonal planes measuring several nanometers, which are basic units of graphite materials. Glass-like carbon is also known as amorphous carbon (or “non-graphitizable carbon) because no graphite structure develops even if heated to around 3000° C.